Broadly, photoacoustic spectroscopy (PAS) is a detection method based on several simple principles: 1) the absorption of light by an analyte molecule; 2) the subsequent generation of an acoustical wave generated by the molecular relaxation, and 3) the detection of the acoustic wave by a pressure sensing device (e.g., a microphone).
The basic photoacoustic effect was discovered over a century ago. A. G. Bell, Proc. Am. Assoc. Adv. Science, Vol. 29, page 115 (1880), Phil Mag. Vol. 11, page 510 (1881; J. Tyndall, Proc. Roy. Soc. Vol. 31, page 307 (1881); and W. C. Rontgen, Phil. Mag. Vol. 11, page 308 (1881) discovered the opto-acoustic effect and its use in the “spectrophone.” Briefly, input optical radiation, periodically interrupted at a frequency in the audible range, was directed upon a gas medium in a glass container; and the periodic pressure fluctuations resulting from the absorption of radiation by the gas was detected by ear through a listening tube connected to the container. Other background references of interest include U.S. Pat. Nos. 3,700,890, issued Oct. 24, 1972 and 3,820,901 issued Jun. 28, 1974, each in the name of L. B. Kreuzer; L. B. Kreuzer, J. Appl. Phys. 42, 2934 (1971); C. F. Dewey et al, Appl. Phys. Letters 23, 633 (1973); R. D. Kamm, J. Appl. Phys. 47, 3550 (1976); E. Max et al, Opt. Comm. 11, 422 (1974); and C. K. Patel et al, Appl. Phys. Letters 30, 578 (1977).
To this day, most photoacoustic cells are macro-scale devices measuring from inches to upward of a meter in length. The basic designs consist of a light source and a sealed cell including gas inlets and outlets, transparent windows, and a sensing microphone. In the past, the optical radiation was provided by an assortment of light sources including lamps, lasers, light-emitting diodes (LED) and even blackbodies. Although some modern trace gas measurement instruments based on photoacoustics utilize lamps, the majority of recent research for trace gas sensing in photoacoustics has been dominated by the use of laser sources. In particular, lasers sources have allowed the added advantage of increased modulation capabilities (up to GHz levels) not possible with other sources. Most thermal sources modulated through alternating current only maintain full modulation depth at low frequency (<100 Hz). Even with mechanical modulation (chopper wheel) of these sources, the highest possible modulation is still modest (<6.4 kHz).
In order to increase sensitivity of the photoacoustic signals, the modulation of the light source is designed to correspond to the acoustic resonant frequency in the photoacoustic cell in order to amplify generated acoustic signals. The lowest order mode that could be acoustically resonated in such a structure would correspond to the first longitudinal mode given by ωres=c/2lres, where ωres is the acoustic frequency, c is the speed of sound, and lres is the resonator length. Resonant photoacoustic cells designed to take advantage of typical MEMS processing would practically have resonant structures on the order of one to several millimeters in length. Assuming atmospheric pressure, ambient temperature, and the resonator lengths suggested above implies an optical radiation source would have to modulated at approximately 10-100 KHz level to drive the photoacoustic cell into acoustic resonance. These requirements exclude the use of any thermal source necessitating the use of either a laser or LED.
Some described devices employ other features to improve sensitivity such as multiple-sensing microphones, resonant acoustic cavities, noise-suppression volumes, turntable light sources and multi-light-pass arrangements. As an example of one improvement, U.S. Pat. No. 4,163,382 discloses a method and apparatus that increases the sensitivity and flexibility of laser optocaustic spectroscopy, with reduced size. According to the method, it was longer as necessary to limit the use of laser optocaustic spectroscopy to species whose absorption must match available laser radiation. Instead, “doping” with a relatively small amount of an optically absorbing gas yields optocaustic signatures of non-absorbent materials (gases, liquids, solids, and aerosols), thus significantly increasing the sensitivity and flexibility of opt caustic spectroscopy.
Another improvement to PAS, called wavelength modulated photoacoustic spectroscopy, or WM-PAS, eliminates a major noise source associated with traditional implementations of PAS. WM-PAS has been practiced in the prior art. An early description of the technique was provided by C. F. Dewey, Optoacoustic Spectroscopy and Detection, (Y-H Pao, ed., Academic Press, New York, 1977), pp. 62-64. Others have since practiced the technique including M. Feher, et al., Applied Optics 33, 1655 (1994); A. Miklos, et al., Applied Physics B 58, 483 (1994); and B. E. R. Olsson, et al., Applied Spectroscopy 49, 1103 (1995). All use sinusoidal wavelength modulation waveforms. U.S. Pat. No. 6,552,792 improves on traditional sinusoidal modulation through the use of a modified square wave to provide increased signal compared to the sinusoidal and triangle waveforms.
Although sensitive, these devices have several shortcomings, including the large size of the cell and other apparatus. Accordingly, there exists the need for a trace chemical sensor, preferably with high-sensitivity, low-cost, low power consumption, and the capability to be mass-produced.